Considering world class sprinters top out at anywhere from 27-30 mph, the fact that that's all they train for, and that Taylor is on turf with gear on not a track with track shoes, I'd say his 20mph is a pretty impressive top speed. But again that's not a comparison to another football player.
After a short search the closes I could find for you is that Taylor (20.5mph) is slower than Michael Vick (21.63mph) and Adrian Peterson (21.60mph).
And btw it does give the other players speed you just have to do a little math, the guy chasing Martinez is running roughly 18.5mph.